e.g, hedrix burning his guitar (its actully this day in history he did that...) at live aid when Bob Dylan broke a guitar string, Ronnie Wood took off his own guitar and gave it to Dylan. Wood was left standing on stage guitarless. After shrugging to the audience, he played air guitar, even mimicking The Who's Pete Townshend by swinging his arm in wide circles, until a stagehand brought him a replacement, ect...

At a Who concert in 1973 (I think), Keith Moon passed out in the middle of their set because he had taken horse tranquilizers before the gig. Then Pete had to ask the audience if there was anyone there who knew how to play the drums. They got some 19-year-old kid to play the drums on several more of the songs during the concert. You've probably heard of that one, but I think it was pretty memorable all right.

in woodstock when the acts that were supposed to play after Richie Havens were late and Richie played everything he had and had nothing left to play, and then improvised Freedom which became one of his greatest hits. epic.
or santana at woodstock, so high on lsd so he thought his guitar neck was a snake.

One weird thing I saw was Jeff Beck approaching the microphone to speak between songs, and he bumped his nose on the mic and knocked it over. I'll never know if it was a gag or not, but it was pretty funny to me.